1. Field of the Invention
The present invention relates to methods for use in fabricating higher-level semiconductor devices with molecular beam epitaxial (MBE) growth. More particularly, this invention pertains to such a method that provides processed wafers of enhanced surface morphology.
2. Description of the Prior Art
The fabrication of higher level integrated circuits involves the selective growth of distinct epitaxial layers on a common substrate. This is due to the fact that different band gap materials and device structures may be called for and distinct active epitaxial regions required when forming, for example, both an FET and a photodetector on a single wafer. It follows that the manufacture of higher level devices requires careful attention to the arrangement and growth of such multiple distinct regions that share extremely small common substrate bases. This is particularly apparent in the very high frequency device range (microwave, millimeter wave and optoelectronic) where high operating frequencies dictate extremely small sizes. Thus precise control of surface morphology, epitaxial layer size, definition and separation are critical.
Compounds of materials from the third (three valence electrons in outer shell) and fifth (five valence electrons in outer shell) columns of the periodic table are often employed as building blocks of high frequency devices. Such materials, known as "III-V compounds" are characterized by high electron mobilities (and, thus, very high frequency operation) and direct band gaps that make them appropriate for optoelectronic applications such as laser diodes and photodetectors. Indium, aluminum and Gallium are useful column III materials while antimony, phosphorous and arsenic are column V elements that are frequently composed and employed in electronically useful III-V compounds.
The fabrication of III-V compound-based higher level devices generally involves the formation of an initial active layer through MBE growth or ion implantation on the III-V compound substrate. The initial active layer is selectively masked by a dielectric layer and exposed portions thereof are then removed by means of a wet chemical etch. A second epitaxial layer is then grown upon the common wafer substrate by an MBE growth process. During this latter process, a residual polycrystalline layer composed of material of the laterformed epitaxial layer is deposited atop remaining portions of the dielectric mask.
The surface morphology of the residual polycrystalline material is generally rough, requiring its removal prior to formation of overlying interconnects. Furthermore, the roughness of such material limits the ability to produce very small feature sizes. Accordingly, selective MBE growth processes require removal of the residual layer of polycrystalline material, a difficult, time consuming and costly step.